1. Field of the Invention
The present invention relates to a coder for an audio signal, a video signal, and the like, and more particularly, to a wavelet transform apparatus for a wavelet coding.
2. Description of the Related Art
Generally, discrete cosine transform (DCT) coding is well known as transform coding, however, this coding has two serious drawbacks, i.e., a blocking effect and a mosquito noise (or corona effect). That is, the blocking effect is mainly derived from the quantization errors in lower frequency coefficients, and the mosquito noise is mainly derived from the quantization errors in higher frequency coefficients. To overcome these two drawbacks, wavelet coding has been developed (see: M. Ohta et al, "Wavelet Picture Coding with Transform Coding Approach", IEICE TRANS. FUNDAMENTALS, Vol. E75-A, No. 7, July 1992).
A prior art wavelet transform apparatus for a one-dimensional signal such as a audio signal includes a plurality of stages of two-band analysis filter circuits, each for performing a two-band analysis filter operation upon a signal to generate a high frequency component signal and a low frequency component signal. In order to carry out such an operation, each of the two-band analysis filter circuits includes a highpass downsampling filter (HPF) and a lowpass downsampling filter (LPF), each having a series of delay units and a convolution calculating circuit. That is, if the number of stages is n, the number of convolution calculating circuits is 2n. This will be explained in detail later.
Similarly, a prior art wavelet transform apparatus for a two-dimensional signal such as a video signal includes a plurality of stages of four-band analysis filter circuits, each for performing a vertical two-band analysis filter operation upon a signal to generate a vertical high frequency component signal and a vertical low frequency component signal, performing a horizontal two-band analysis filter operation upon the vertical high frequency component signal to generate a horizontal high vertical high frequency component signal and a horizontal low vertical high frequency component signal, and performing a horizontal tow-band analysis filter operation upon the vertical low frequency component signal to generate a horizontal high vertical low frequency component signal and a horizontal low vertical low frequency component signal. In order to carry out such operations, each of the four-band analysis filter circuits includes a vertical two-split subband filter formed by a vertical highpass filter (VHPF) and a vertical lowpass filter (VLPF), a horizontal two-split subband filter formed by a horizontal highpass filter (HHPF) and a horizontal lowpass filter (HLPF), and another horizontal two-split subband filter formed by a horizontal highpass filter (HHPF) and a horizontal lowpass filter (HLPF). Also, in this case, each of the filters has a series of delay units and a convolution calculating circuit. That is, if the number of stages is n, the number of convolution calculating circuits is 6n. This will be explained in detail later.
However, the above-mentioned prior art wavelet transform apparatus is large in size, due to the large number of filters (i.e., convolution calculating circuits). Particularly, the large number of multipliers included in the convolution calculating circuits increases the size of the wavelet transform apparatus.